Reproductive History Reconstructions of Black Bears

Methods

The years in which a female black bear has successfully reared cubs can sometimes be identified by the presence of characteristic indicators in tooth cementum. Cub rearing is accompanied by a diminished production of the more abundant, lightly staining cementum of the spring and summer growing seasons. The reduced cementum growth creates a thinned cementum layer that can be identified because female bears do not rear cubs during successive years, but during alternate ones. The thinned cementum of a cub rearing year alternates with a “rebound”, thickened layer following. In successive years, the reproductively successful black bear has alternating thin and thick light cementum layers, indicating “cub rearing” and “no cub rearing”, respectively.

Black bear PM1 tooth section; cementum age 13 years; Month of kill June. Matson’s uses two criteria to identify cub rearing: 1) Marked thinning of the light cementum layer, which is formed during the season of somatic growth, during the cub rearing year; 2) Rebound thickening of the light cementum layer during the growth season following the cub rearing year. The illustrated tooth section has strong evidence, with both criteria being present, for cub rearing at ages 4, 6, 8, and 10. Only one of the two criteria, light cementum layer thinning, is present at age 12 and the evidence for cub rearing at this age is incomplete.

Applications and Complications

The identification of cub rearing years permits an estimate of the age of first reproduction and reproductive potential. However, not all reproductively active female black bears have cementum with identifiable cub year indicators. Indicators often become less evident with age, preventing a full assessment of the older female bear’s reproductive history even though indicators are present in younger years.

To date, the technique appears to be only useful for black bears, and not other bear species.

Recommendations

Treat reproductive history reconstructions as approximations. The cementum indicators for the first years of cub rearing are usually the most evident ones, and therefore early reproductive history may yield the most useful data from the method. Use the largest possible sample size to estimate the population’s reproductive potential based upon cementum evidence. The estimate will undoubtedly be low, because indicators are less visible (and likely) more often missed in the cementum of older bears.

Brown Bear Reproductive Histories

Matson’s studied the potential for reconstructing reproductive histories in brown bears. Photographs were used to directly measure cementum layer thicknesses, which were then tabulated for mathematical calculation to detect differences that otherwise were not visually evident. Among the 29 female brown bears with part or all of their reproductive histories known, we correctly identified only 13 of 62 known cub years. Because of this low accuracy, we concluded that brown bear reproductive histories cannot be reconstructed using the method shown to be successful for black bears. Our findings were presented in a poster at the Eleventh International Conference on Bear Research and Management, Gatlinburg, Tennessee. Please see Publications page of this site, Matson et al. 1998.